In this article we discuss the following questions: what is the key to cross-border cooperation, particularly in the case of European mountain regions? Does the legal framework represent a starting point for the development of cross-border relations or is it just a simple tool in the service of territorial thinking? If we consider that there is a strong link between cross-border relations and the European project, the above questions also raise the issue of the relative roles of the bottom-up and top-down processes in the dynamics of European integration. Results suggest that the legal framework represents an indispensable tool in the service of territorial thinking but it cannot be a starting point for the development of cross-border relations. The solutions do not only concern the law but also territory; the essential question is how to define the social, political and spatial boundaries of cross-border cooperation.